1. Field of the Invention
The present invention relates to a switching element using electrochemical reaction, a programmable logic integrated circuit and a memory element.
2. Description of the Related Art
Today, electronic apparatuses and the like use numbers of integrated circuits. Numbers of integrated circuits used in an electronic apparatus are so-called ASIC (Application Specific Integrated Circuit) which are dedicated circuits designed for the electronic apparatus. With such an application specific integrated circuit, since arrangement of a logic cell (a logic circuit as a unit of an AND circuit, an OR circuit and the like) and connection of logic cells are made in an integrated circuit manufacturing process, a circuit structure can not be modified after manufacturing.
In recent years, competition for developing electronic apparatuses has heated up and down-sizing of electronic apparatuses is accelerated. Under these circumstances, a programmable logic (programmable logic integrated circuit) draws attention which enables one chip to select a specific function from among many functions by changing a circuit structure by an electronic signal even after manufacturing. Programmable logic is structured to have a plurality of logic cells connected with each other by a switch. Representative examples of programmable logics include FPGA (Field-Programmable Gate Array) and DRP (Dynamically Reconfigurable Processor).
Although programmable logics thus attract attention, mounting programmable logics on electronic apparatuses and the like is limited so far. The reason is as follows. In programmable logics realized so far, the size of a switch connecting logic cells is large to have a large on-resistance. In order to limit the number of such switches disposed, logic cells having a large number of transistors are arranged in small number. As a result, flexibility in combining logic cells is reduced to limit a function which can be provided by a programmable logic. In other words, largeness in size of a switch used in a conventional programmable logic and its largeness in on-resistance limit the function of the programmable logic to limit mounting of the programmable logic on an electronic apparatus or the like.
In order to vary functions of a programmable logic to promote its mounting on an electronic apparatus or the like, it is necessary to reduce the size of a switch connecting logic cells with each other to decrease its on-resistance. Disclosed as a switch meeting such requirements is a switching element using metal ion migration within ion conductor (solid in which ions can move freely) and electrochemical reaction (see e.g. International Publication WO 2003/094227 A1 (Literature 1)). The switching element disclosed in the literature has a smaller size and a smaller on-resistance than those of a semiconductor switch (MOSFET etc.) which has been widely used in conventional programmable logics.
FIG. 8 is a sectional schematic diagram showing one example of a structure of the switching element disclosed in International Publication WO 2003/094227 A1 (Literature 1). The switching element is structured to have a first electrode 11 and a second electrode 12 provided on the first electrode 11 with an ion conductor layer 13 (represented as solid electrolyte in Literature 1) provided therebetween. The ion conductor layer 13 is a medium in which metal ions can transport.
Operation of the switch shown in FIG. 8 will be described.
With the second electrode 12 grounded, when a negative voltage is applied to the first electrode 11, metal of the second electrode 12 is dissolved as metal ions into the ion conductor layer 13. Then, the metal ions in the ion conductor layer 13 are deposited as metal on the surface of the first electrode 12 and the deposited metal forms metal dendrite which connects the first electrode 11 and the second electrode 12. Metal dendrite is metal deposition as deposition of metal ions in the ion conductor layer 13. Electrically connecting the first electrode 11 and the second electrode 12 by metal dendrite causes the switch to turn on.
On the other hand, with the second electrode 12 in the on state grounded, when a positive voltage is applied to the first electrode 11, metal dendrite is dissolved into the ion conductor layer 13, so that a part of the metal dendrite is cut out. As a result, electrical connection between the first electrode 11 and the second electrode 12 is cut off to turn off the switch. From a stage preceding to a stage where the electrical connection is completely cut off, electrical characteristics are changed such as resistance between the first electrode 11 and the second electrode 12 is increased or interelectrode capacitance is changed to ultimately cut off electrical connection. As a material of the first electrode 11, preferable is one that fails to supply metal ions into the ion conductor layer at the application of a voltage. In addition, for changing from the above-described off state to the on state, another application of a negative voltage to the first electrode 11 is required.
Use of the switching element shown in FIG. 8 as a switch for switching programmable device interconnection is proposed (Journal of Solid State Circuits, vol. 40, No. 1, pp. 168-176, 2005 (Literature 2)). As compared with a conventional type, using this switching element not only reduces a switch area to 1/30 and switch resistance to 1/50 but also enables fabrication of a switching element into an interconnection layer. Therefore, reduction in a chip area and improvement in an interconnection delay can be expected. Furthermore, since a logic cell of a programmable logic can be reduced, efficiency of circuit use can be drastically improved to result in reducing the chip area to 1/10 to triple power efficiency. While a conventional type programmable logic has limited application due to its large chip size and low power efficiency, a new programmable logic using this switching element can cover a wide range of application.
Literature 1: International Publication WO 2003/094227 A1
Literature 2: Journal of Solid State Circuits, vol. 40, No. 1, pp. 168-176, 2005
The switching element disclosed in Literature 1 is realized by using Cu/Cu2S and Ag/Ag2S or the like for the combination of an electrode material and an ion conductor layer. In any of the combinations of materials, a switching voltage as a voltage to be applied to a first electrode for making transition of the switching element from the on state to the off state or from the off state to the on state is approximately from 0.05V to 0.3 V. On the other hand, as to a logic signal used as a signal in a programmable logic, in general, a voltage indicative of one information of two kinds of information is Vdd as an operating voltage of a logic circuit and a voltage indicative of the other information is 0V. Vdd used today in a silicon integrated circuit is approximately from 1 to 2 V.
Switching voltage is 0.3 V to the highest as described above and when Vdd of the logic signal is 1.0V, it will be lower than Vdd. Therefore, every time a logic signal with the voltage Vdd is applied to the switching element, 1.0 V voltage is applied to the first electrode, so that the state of the switch might be changed by the logic signal itself. In this case, the switching element might not function as a switch. It is necessary to more stabilize the switching element by increasing a switching voltage.
In addition, as a time period for maintaining the state of the switching element (time period for maintaining non-volatility), a period longer than a product life (10 years in general) of a programmable logic used is required. Since thermal energy in the vicinity of room temperature is 26 meV, as the switching voltage nears 26 mV, a probability of spontaneous state transition due to thermal noise will be increased. For elongating a maintaining time period, the switching voltage should be increased.